There is a conventional mobile communication system which performs send/receive of ATM cells utilizing an ATM line between a higher rank station and a base station.
FIG. 1 is a diagram showing an example of the construction of a conventional mobile communication system. In this mobile communication system, a base station A1 and a base station B2 are connected to a higher rank station through a network 3. The network 3 is connected to the base station through a leased line. Trailing data from the higher rank station is sent to the base station A1 through a leased line and to the base station B2 through another leased line. Data received by the base station A is designated as trailing ATM cell A4, and data sent from the base station A to the higher rank station is designated as leading ATM cell A6. Likewise, for the base station B, data received by the base station B is designated as trailing ATM cell B5, and data sent from the base station B to the higher rank station is designated as leading ATM cell B7.
FIG. 6 shows an ATM cell format. An ATM cell 25 is constituted by data of 53 bytes. In this case, 5 bytes from the head constitute an ATM header 26, and the remaining 48 bytes constitute a payload 27. The ATM header 26 comprises GFC (generic flow control) 28, VPI (virtual path identification) 29, VCI (virtual channel identification) 30, PT (payload type) 31, CLP (cell loss priority) 32, and HEC (header error control) 33.
GFC 28 is provided for flow control which is performed when traffic has been increased and, consequently, overload state has taken place over the network 3. VPI 29 is utilized in setting of a virtual path between the higher rank station and the base station, and VCI 30 is utilized for the identification of each of a plurality of data in the set VP (virtual path) in the communication of the plurality of data. PT 31 indicates the state of cell (congestion), and CLP 32 indicates the significance of cell. HEC 33 functions to detect bit error of the ATM header 26, and indicates the results of coding of 8-bit CRC for 4 bytes in the header except for HEC 33. A payload 27 indicates the storage region of communication data.
FIG. 7 is a diagram showing a leased line (a secondary group) frame format as one example of the leased line frame format. The secondary group of leased lines have a transmission capacity of 6.3 Mbps wherein 789 bits are arranged in a frame of time length 125 μs. In this format, 8 bit-length 98 TSs (time slots) can be obtained. In a portion of 96 TSs (96 bytes×8=768 bits) among them, ATM cells 25 are continuously arranged.
FIG. 2 shows an example of the construction of a conventional trailing ATM cell. The trailing ATM A4 from the higher rank station is received in an HWY interface section 8 in the base station A1. A receive processor 35 has the function of terminating the trailing ATM cell A4. The base station A1 and the base station B2 are handled as devices which are independent of each other. Thus, these base stations have the same construction.
FIG. 3 shows an example of the construction of a conventional trailing processing function. This function will be explained by taking the base station A1 as an example. For data received from the leased line, the physical layer is terminated in the HWY interface section 8. In a leased line frame trailer 11, the frame format shown in FIG. 7 is synchronized. After the establishment of the synchronization of the frame, cell synchronization for establishing the position of cell boundary is carried out in an ATM cell synchronous detector 12, for identifying an ATM cell 25 mapped within the frame format.
For the ATM cell 25 within the frame format which has been picked out in the ATM cell synchronous detector 12, error detection of HEC 33 is carried out in the ATM HEC error detector 22, and the ATM cell 25, which has been found to have an error, is discarded in this function. The trailing ATM cell A4. which has been judged to be effective in the physical layer is sent to the receive processor 35. In the receive processor 35, VPI 29 is confirmed for the trailing ATM cell A4 received by VPI filter 14, and only the trailing ATM cell A4 having VPI 29 assigned to the base station A1 is transferred to the next processing. The ATM cell 25, wherein VPI 29 is different from the set value, is discarded by this function.
The trailing ATM cell A4, which has been passed through the filter of VPI 14, judges various ATM cells assigned by the VCI filter 15, and is terminated at an ATM cell trailer 16.
FIG. 4 shows an example of the construction of a conventional leading ATM cell. The leading ATM cell A6 from the base station A is generated in an ATM cell generator 20. Since the base station A1 and the higher rank station are connected to each other through a leased line, the base station A1 maps the leading ATM cell B7 in the leased line (secondary group) frame format shown in FIG. 7 through the use of a frame generator 23. Further, there is also a function of mapping of an idol cell 34 in the frame format according to the transmission rate capacity of the leading ATM cell A6.
Regarding another example of the conventional mobile communication system, for example, Japanese Patent No. 3003779 proposes a mobile communication system wherein the control link between an exchange and a radio base station is established using a control signal by ATM.
The above-described conventional prior art techniques, however, had the following problems.
The first problem is such that when reduction in size/reduction in capacity of the base station has lead to a reduction in quantity of data for communication between the base station and the higher rank station and, consequently, the quantity of data has become smaller than the capacity of the leased line, in the mapping within the leased line, the proportion of the effective cell becomes larger than the proportion of the idol cell. This creates wasteful line charge. When the number of users to be supported by the base station has been changed, for example, due to a reduction in size, changing the rate including the quantity of data in the cable to an optimal rate is preferred. Since, however, there is no leased line suitable for the capacity, the system of the base station should be constructed in such a state that an unused band exists.
The second problem is such that, in solving the first problem, the connection of the conventional model should be possible. That is, a system should be devised which has succeeded to the conventional system design and can utilize existing models.